(1) Technical Field
This invention relates to electronic radio frequency (RF) circuits, and more particularly to RF signal switching circuits.
(2) Background
Radio frequency (RF) signal switches are common circuits for routing RF signals between various parts of an RF system, such as between one or more antennas and one or more transmitter and/or receiver circuits. For example, RF signal switches are used in such devices as one-way and two-way radios, including cellular telephones, and in global positioning system (GPS) devices.
It is fairly common to use RF signal switches in electronic systems that are sensitive to noise, such as radio signal receivers. Such receivers are often presented with very weak signals (for example, transmitted radio waves captured by an antenna), and accordingly, it is common to use a low-noise amplifier (LNA) to amplify such signals. It is also common to provide a matching network at the input of the LNA to maximize power transfer to the LNA.
The noise figure (NF) of a low noise amplifier (LNA) is very sensitive to the impedance presented and the insertion loss (IL) in front of the input to the LNA. For example, in a typical cellular radio LNA configuration, a set of bandpass filters in series with an RF switch exist at the input of the LNA and are used to select a specific band to be presented to a radio transceiver. In order to reduce the NF, RF signal switches with low insertion loss (IL) are required. However, in some modes of operation, the LNA must be bypassable, and in such a mode, the LNA should be isolated from the bypass signal path.
For example, FIG. 1 is a schematic diagram of a prior art RF signal switch 100 in which one of N RF input terminals RF1-RFN may be connected to a Switch Output port, either through a matching network and LNA block 102 or through a bypass path 104. The RF input terminals RF1-RFN may be connected to respective antennas; the matching network and LNA block 102 are generally external to the RF signal switch 100, as indicated by the dashed lines.
In the illustrated example, in an “in-circuit” mode, the input terminal RF1 may be connected to the matching network and LNA block 102 by closing the corresponding series switch S1, opening a corresponding shunt switch Sh1, opening the Bypass Switch, closing an output connector switch SC, and opening an output connector shunt switch ShC. In this mode, for each other RF input terminal RF2-RFN, the corresponding series switch S2-SN is open and the corresponding shunt switch S2-SN is closed. Any other RF input terminal RF2-RFN may be connected in-circuit to the matching network and LNA block 102 in a similar manner.
In a bypass mode, the input terminal RF1 may be connected directly to the Switch Output by closing the corresponding series switch S1, opening the corresponding shunt switch Sh1, closing the Bypass Switch, opening the output connector switch SC, and closing the output connector shunt switch ShC. In this mode, for each other RF input terminal RF2-RFN, the corresponding series switch S2-SN is open and the corresponding shunt switch S2-SN is closed. Any other RF input terminal RF2-RFN may be connected to bypass the matching network and LNA block 102 in a similar manner.
A problem with the circuit configuration shown in FIG. 1 is that the matching network at the input of the LNA is not isolated from the bypass path 104 when the LNA is not active. The matching network can adversely affect the output impedance of the RF signal switch 100 and cause the IL of the bypass path 104 to be degraded, the severity of which is dependent on the design and component values of the LNA input matching network. This degradation of the bypass path IL causes module designers to struggle to find a balance between having a good LNA input match and maintaining a low bypass path IL.
If it is desired to isolate the bypass path from the LNA input matching network, standard practice is to add another series/shunt switch pair before the matching network. For example, FIG. 2 is a schematic diagram of the prior art RF signal switch of FIG. 1 in which an isolation series/shunt switch pair S1/Sh1 106 are connected before the matching network and LNA block 102. In the bypass mode, the isolation series switch S1 is open and the isolation shunt switch Sh1 is closed, thereby coupling the matching network input to circuit ground and completely isolating the matching network from the bypass path 104. Conversely, in the in-circuit mode, the isolation series switch S1 is closed and the isolation shunt switch Sh1 is open.
However, a downside of the circuit configuration shown in FIG. 2 is that the added series switch S1 increases the input IL to the LNA compared to the configuration shown in FIG. 1, and thus increases the system NF. This is so because the illustrated switches are not perfect conductors when closed, but exhibit some amount of impedance.
Accordingly, there is a need for an RF signal switching circuit that simultaneously exhibits low input IL (and thus a low NF) and high isolation. The present invention addresses this need.